Although it will become evident to those skilled in the art that the present invention is applicable to a variety of implantable medical devices utilizing pulse generators to stimulate selected body tissue, the invention and its background will be described principally in the context of a specific example of such devices, namely, cardiac pacemakers or defibrillators for providing precisely controlled stimulation pulses to the heart. However, the appended claims are not intended to be limited to any specific example or embodiment described herein.
Cardiac pacemakers, and other implantable medical devices such as cardiac defibrillators, are hermetically packaged to isolate the device from the body environment. Such devices require that electrical signals be passed between the packaged device and its external connectors, without compromising the hermeticity of the package.
Typically, electrical coupling between the electronic circuits of the implantable medical device and the external connections provided by a connector assembly mounted outside of the implantable device are provided by a feedthrough assembly. The feedthrough assembly extends through the hermetically sealed outer wall of the device and into the connector assembly so as to couple the electronic circuits within the implantable device to lead-receiving receptacles within the connector assembly. A conductive path is provided through the feedthrough by a conductor pin which is electrically insulated from the container. Many such feedthroughs are known in the art which provide the electrical path and seal the electrical container from its ambient environment.
Such electrical devices can, under some circumstances, be susceptible to electromagnetic interference (EMI). At certain frequencies for example, EMI can inhibit pacing in an implantable medical device. This problem has been addressed by incorporating a capacitor structure within the feedthrough ferrule, thus shunting any EMI at the entrance to the implantable device for high frequencies. This has been accomplished with the aforementioned capacitor structure by combining it with the feedthrough and incorporating it directly into the feedthrough ferrule. Typically, the capacitor electrically contacts the pin lead and the ferrule.
In one approach, a filter capacitor is combined directly with a terminal pin assembly to decouple interference signals to the housing of the medical device. In a typical construction, a coaxial feedthrough filter capacitor is connected to a feedthrough assembly to suppress and decouple undesired interference or noise transmission along a terminal pin.
So-called discoidal capacitors having two sets of electrode plates embedded in spaced relation within an insulative substrate or base typically form a ceramic monolith in such capacitors. One set of the electrode plates is electrically connected at an inner diameter surface of the discoidal structure to the conductive terminal pin utilized to pass the desired electrical signal or signals. The other or second set of electrode plates is coupled at an outer diameter surface of the discoidal capacitor directly or indirectly (for example, via a ground lead) to a cylindrical ferrule of conductive material, wherein the ferrule is electrically connected in turn to the conductive housing or case of the implantable medical device.
Feedthrough capacitors of this general type are commonly employed in implantable pacemakers, defibrillators and the like, wherein a device housing is constructed from a conductive biocompatible metal such as titanium and is electrically coupled to the feedthrough filter capacitor. The filter capacitor and terminal pin assembly prevent interference signals from entering the interior of the device housing, where such interference signals might otherwise adversely affect a desired function such as pacing or defibrillating.
Although feedthrough filter capacitor assemblies of the types described above have performed in a generally satisfactory manner, it would be advantageous to be able to increase the number of lead connections while minimizing the footprint of the existing feedthrough layout and reducing cost of manufacture.